Unfortunatly with that unit, it is not possible to use a center channel with the current pre-outs that it provides. I would have to guess that the center channel signal was mixed into the front speakers as there are only fronts rears n sub outputs. I would assume that the only way to get the true signal for center channel dts is to use the digital optical ouput.
your other option for a center channel speaker is to get a small amplifier that you can input the front speaker preouts and can give you a mono (or bridgeable) output that would match the speakers power handling.

"In the beginners mind there are many possibilities, but in the experts mind there are few."- Shunryu Suzuki
"Do it right or don't do it at all"

Yeah, Sorry. I tend to over-complicate things sometimes. force of habit with my customers that they want me to explain to them how to do things instead of actually paying for me to do it.
I'm not trying to confuse on purpose. Hopefully you can understand.

"In the beginners mind there are many possibilities, but in the experts mind there are few."- Shunryu Suzuki
"Do it right or don't do it at all"

hehe, how about this: WHY do you want a center channel? Why not keep it simple at 4.1 instead of spending a few hundred for 5.1 for an volume the size of your shower? Do you really think you'll tell the difference?